关于Linux 网络抓包的一些笔记整理

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内容摘要：

文章介绍了Linux中常用的网络抓包工具tcpdump的基本用法和过滤条件，并通过HTTP/TCP和ICMP(ping)请求的抓包分析，以及Nginx的用户认证抓包分析，展示了抓包在网络问题排查中的重要性。

当前摘要为AI生成

* 遇到一个 ping 单通 的情况，需要抓包分析下，所以整理这部分笔记
* 博文内容涉及：
  * HTTP/TCP 抓包分析 Demo
  * ICMP 抓包分析 Demo
  * Nginx 抓包分析用户名密码 Demo
  
## tcpdump 简单介绍

在 Linux 中常常使用 tcpdump 网络抓包工具来进行抓包，排查一些网络问题,需要安装的工具包
```shell
┌──[root@vms82.liruilongs.github.io]-[~]
└─$rpm -ql tcpdump || yum -y install tcpdump
```
tcpdump 抓包命令基本用法 tcpdump [选项] [过滤条件]常见监控选项

* -i 指定监控的网络接口,指定抓取某一个网卡接口的流量；不使用-i 指定网卡，tcpdump 会默认抓取第一块网卡的流量；-i any 监控抓取的是所有网卡的流量；
* -A 转换为ACSII码，以方便阅读
* -w 将数据包信息保存到指定文件
* -r 从指定文件读取数据包信息
* -c 定义抓包个数 (默认会一直抓取下去)
* -nn 不解析域名

tcpdump的过滤条件，按Ctrl + C 键停止抓包

* 类型：host、net、port 80、portrange 600-800
* 方向：src(从指定IP接收)、dst(发送到指定ip)
* 协议：tcp、udp、ip、wlan、arp、......
* 多个条件组合：and、or、not

## 实战

### HTTP/TCP 请求抓包分析

使用 python2 的 SimpleHTTPServer 模块启动一个 简单的 HTTP 服务

```shell
[root@vms152 ~]# jobs
[1]+  运行中               coproc COPROC python -m SimpleHTTPServer 8080 &
```
 发出HTTP请求 
 ```shell
┌──[root@vms152.liruilongs.github.io]-[~]
└─$curl 192.168.29.152:8080
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html>
<title>Directory listing for /</title>
<body>
。。。。。
```
抓包命令 tcpdump -i ens32 -A -nn -w /root/web.cap host 192.168.29.152 and port 8080抓取 ens32网卡，主机为 192.168.29.152 端口为 8080 的所有出站入站的包。抓包写入 /root/web.cap

```shell
┌──[root@vms152.liruilongs.github.io]-[~]
└─$tcpdump -i ens32  -A -nn -w /root/web.cap  host 192.168.29.152 and port 8080
tcpdump: listening on ens32, link-type EN10MB (Ethernet), capture size 262144 bytes
^C11 packets captured
11 packets received by filter
0 packets dropped by kernel
```
抓取到 host 192.168.29.152 and port 8080 的所有包，这里数据包包括出站和入站的所有包，没有指定协议即，即包括所有

tcpdump 的数据包文件是是乱码，Linux 下需要通过 -r 命令来读取 tcpdump -A -r /root/web.cap
```shell
┌──[root@vms152.liruilongs.github.io]-[~]
└─$tcpdump -A -r /root/web.cap
reading from file /root/web.cap, link-type EN10MB (Ethernet)
03:24:14.285274 IP vms152.liruilongs.github.io.41204 > 192.168.29.152.webcache: Flags [S], seq 317480493, win 29200, options [mss 1460,sackOK,TS val 270896233 ecr 0,nop,wscale 7], length 0
E..<.D@.@.................^-......r............
.%.i........
03:24:14.286103 IP 192.168.29.152.webcache > vms152.liruilongs.github.io.41204: Flags [S.], seq 601235906, ack 317480494, win 64240, options [mss 1460], length 0
E..,9.....HJ............#.!...^.`...lm........
03:24:14.286137 IP vms152.liruilongs.github.io.41204 > 192.168.29.152.webcache: Flags [.], ack 1, win 29200, length 0
E..(.E@.@..     ..............^.#.!.P.r.....
03:24:14.286282 IP vms152.liruilongs.github.io.41204 > 192.168.29.152.webcache: Flags [P.], seq 1:84, ack 1, win 29200, length 83: HTTP: GET / HTTP/1.1
E..{.F@.@.................^.#.!.P.r.....GET / HTTP/1.1
User-Agent: curl/7.29.0
Host: 192.168.29.152:8080
Accept: */*

03:24:14.286480 IP 192.168.29.152.webcache > vms152.liruilongs.github.io.41204: Flags [.], ack 84, win 64240, length 0
E..(9.....HM............#.!...^.P.............
03:24:14.287219 IP 192.168.29.152.webcache > vms152.liruilongs.github.io.41204: Flags [P.], seq 1:18, ack 84, win 64240, length 17: HTTP: HTTP/1.0 200 OK
E..99.....H;............#.!...^.P.......HTTP/1.0 200 OK

03:24:14.287228 IP vms152.liruilongs.github.io.41204 > 192.168.29.152.webcache: Flags [.], ack 18, win 29200, length 0
E..(.G@.@.................^.#.!.P.r.....
03:24:14.287349 IP 192.168.29.152.webcache > vms152.liruilongs.github.io.41204: Flags [FP.], seq 18:913, ack 84, win 64240, length 895: HTTP
E...9.....D.............#.!...^.P...@...Server: SimpleHTTP/0.6 Python/2.7.5
Date: Mon, 19 Dec 2022 19:24:14 GMT
Content-type: text/html; charset=UTF-8
Content-Length: 758

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html>
<title>Directory listing for /</title>
<body>
。。。。。。。
</body>
</html>

03:24:14.287355 IP vms152.liruilongs.github.io.41204 > 192.168.29.152.webcache: Flags [.], ack 914, win 30430, length 0
E..(.H@.@.................^.#.%TP.v.....
03:24:14.287560 IP vms152.liruilongs.github.io.41204 > 192.168.29.152.webcache: Flags [F.], seq 84, ack 914, win 30430, length 0
E..(.I@.@.................^.#.%TP.v.....
03:24:14.287692 IP 192.168.29.152.webcache > vms152.liruilongs.github.io.41204: Flags [.], ack 85, win 64239, length 0
E..(9.....HJ............#.%T..^.P....F........
```

通过上面的抓包数据，可以看到一次 完整的 TCP 的握手挥手。上面的数据直接看不太方便，所以需要一些工具来分析包内数据，可以通过 wireshark 分析抓取的数据包，导出文件用 wireshark 打开 
 ![](/home/attachment/202308/03/1263871_169102665646239.png)
 
  可以看到，前三个包为一次 TCP 握手建立连接，第四个包为 建立 TCP 连接后的客户端发出 HTTP 请求(这个时候也有一次 ACK )，之后的三次为 数据传输的过程，然后是 HTTP 响应(FIN/ACK),四次挥手的，这里看到三次，是因为在 HTTP 协议返回的时候，携带了第一次挥手的报文，在抓包的时候我们没有指定tcp协议，所以看到三次 
  
  ![](/home/attachment/202308/03/1263871_169102667636793.png)
  
   指定协议抓包可以更清楚的看到三次握手，四次挥手 
   ```shell
┌──[root@vms152.liruilongs.github.io]-[~]
└─$tcpdump -i ens32  -A -nn host 192.168.29.152 and port 8080 and tcp
```
 同时可以通过抓包获取传输的数据 
 ![](/home/attachment/202308/03/1263871_169102677520406.png)

### ICMP(ping) 请求抓包分析

有的时候，网络不通，可以通过 抓包的方式来确认是出站的问题，还是入站的问题

通过 192.168.29.152机器 去 ping 机器 192.168.26.152 ，网络不通，一直超时，说明包进去了，但是没有回来
```shell
[root@vms152 ~]# ip a | grep gl
    inet 192.168.29.152/24 brd 192.168.29.255 scope global ens32
[root@vms152 ~]# ping -i 0.01 -c 3 192.168.26.152
PING 192.168.26.152 (192.168.26.152) 56(84) bytes of data.

--- 192.168.26.152 ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 70ms

[root@vms152 ~]#
```
 但是通过 192.168.26.152 去 ping 机器 192.168.29.152 确是通的，想确认是那里问题 
 ```shell
┌──[root@vms152.liruilongs.github.io]-[~]
└─$ip a | grep gl
    inet 192.168.26.152/24 brd 192.168.26.255 scope global ens32
┌──[root@vms152.liruilongs.github.io]-[~]
└─$ping -i 0.01 -c 3 192.168.29.152
PING 192.168.29.152 (192.168.29.152) 56(84) bytes of data.
64 bytes from 192.168.29.152: icmp_seq=1 ttl=128 time=0.907 ms
64 bytes from 192.168.29.152: icmp_seq=2 ttl=128 time=1.03 ms
64 bytes from 192.168.29.152: icmp_seq=3 ttl=128 time=0.991 ms

--- 192.168.29.152 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 20ms
rtt min/avg/max/mdev = 0.907/0.977/1.033/0.052 ms
┌──[root@vms152.liruilongs.github.io]-[~]
└─$
```
29.152 和 26.152 不同网段通信，需要路由实现，当前我们通过 Linux 的软路由功能实现,下面为软路由的机器，为了方便，这里称为代理机器或者网关机器。 
```shell
┌──[root@vms.154.liruilongs.github.io]-[~]
└─$ip a | grep gl
    inet 192.168.29.154/24 brd 192.168.29.255 scope global ens32
    inet 192.168.26.154/24 brd 192.168.26.255 scope global ens33
┌──[root@vms.154.liruilongs.github.io]-[~]
└─$
```

在 26.152 对 29.152进行抓包

```shell
┌──[root@vms152.liruilongs.github.io]-[~]
└─$tcpdump -i ens32  -A -nn  host 192.168.29.152 and icmp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
```
 同时在 29.152 发起对 26.152 的 ping 请求 
 ```shell
[root@vms152 ~]# ping -i 0.01 -c 3 192.168.26.152
PING 192.168.26.152 (192.168.26.152) 56(84) bytes of data.

--- 192.168.26.152 ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 37ms

[root@vms152 ~]#
```

可以看到，可以收到 29.152 的 ping 的请求 ICMP echo request ，并且做出了响应 ICMP echo reply
 ```shell
┌──[root@vms152.liruilongs.github.io]-[~]
└─$tcpdump -i ens32  -A -nn  host 192.168.29.152 and icmp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ens32, link-type EN10MB (Ethernet), capture size 262144 bytes
04:35:24.595447 IP 192.168.29.152 > 192.168.26.152: ICMP echo request, id 13245, seq 1, length 64
E..TL.@.?.5...........~.3......c.....$...................... !"#$%&'()*+,-./01234567
04:35:24.595485 IP 192.168.26.152 > 192.168.29.152: ICMP echo reply, id 13245, seq 1, length 64
E..T2...@...............3......c.....$...................... !"#$%&'()*+,-./01234567
04:35:24.607313 IP 192.168.29.152 > 192.168.26.152: ICMP echo request, id 13245, seq 2, length 64
E..TL.@.?.5.............3......c.....R...................... !"#$%&'()*+,-./01234567
04:35:24.607344 IP 192.168.26.152 > 192.168.29.152: ICMP echo reply, id 13245, seq 2, length 64
E..T2...@...............3......c.....R...................... !"#$%&'()*+,-./01234567
04:35:24.635735 IP 192.168.29.152 > 192.168.26.152: ICMP echo request, id 13245, seq 3, length 64
E..TL.@.?.5o............3......c....~....................... !"#$%&'()*+,-./01234567
04:35:24.635783 IP 192.168.26.152 > 192.168.29.152: ICMP echo reply, id 13245, seq 3, length 64
E..T2&..@...............3......c....~....................... !"#$%&'()*+,-./01234567
```
 说明问题不在这里，是其他机器的问题, 可能出现在软路由的机器上面,这次在 软路由的机器上面 抓包分析 
 ```shell
┌──[root@vms.154.liruilongs.github.io]-[~]
└─$tcpdump -i ens32  -A -nn  host 192.168.29.152 and icmp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ens32, link-type EN10MB (Ethernet), capture size 262144 bytes
04:42:23.905630 IP 192.168.29.152 > 192.168.26.152: ICMP echo request, id 13246, seq 1, length 64
E..TY.@.@.(............P3......c....#....................... !"#$%&'()*+,-./01234567
04:42:23.925388 IP 192.168.29.152 > 192.168.26.152: ICMP echo request, id 13246, seq 2, length 64
E..TY#@.@.(.............3......c.....9...................... !"#$%&'()*+,-./01234567
04:42:23.950773 IP 192.168.29.152 > 192.168.26.152: ICMP echo request, id 13246, seq 3, length 64
E..TY&@.@.(...........;.3......c....s....................... !"#$%&'()*+,-./01234567
^C
3 packets captured
3 packets received by filter
0 packets dropped by kernel
```
可以看到，在ens32 网卡，只有 ping 请求的包 出现 ICMP echo request ,没有响应的包出现。即问题出现在这里，29.152 ping 发送的包,确实到了 26.152，并且确实从 26.152返回了包，但是返回之后找不到回家的路，不知道对应的IP在那里。

不同网段互通需要路由器，这里 26.152 的网关配置为 26.2 ,但是 26.2 不是作为路由器的机器，所以没有跨网段的能力，也没有对应的路由控制表，所以 26.152 的网关需要配置为 26.154 。

```shell
[root@vms152 ~]# ping -i 0.01 -c 4 192.168.26.152
PING 192.168.26.152 (192.168.26.152) 56(84) bytes of data.
64 bytes from 192.168.26.152: icmp_seq=1 ttl=63 time=1.13 ms
64 bytes from 192.168.26.152: icmp_seq=2 ttl=63 time=0.875 ms
64 bytes from 192.168.26.152: icmp_seq=3 ttl=63 time=0.854 ms
64 bytes from 192.168.26.152: icmp_seq=4 ttl=63 time=4.00 ms

--- 192.168.26.152 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 31ms
rtt min/avg/max/mdev = 0.854/1.715/4.001/1.324 ms
[root@vms152 ~]#
```

同样抓包可以确认 
 ```shell
┌──[root@vms.154.liruilongs.github.io]-[~]
└─$tcpdump -i ens32  -A -nn host 192.168.29.152 and icmp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ens32, link-type EN10MB (Ethernet), capture size 262144 bytes
22:37:01.732511 IP 192.168.29.152 > 192.168.26.152: ICMP echo request, id 13818, seq 31, length 64
E..T..@.@.............h.5......c....e/...................... !"#$%&'()*+,-./01234567
22:37:01.733040 IP 192.168.26.152 > 192.168.29.152: ICMP echo reply, id 13818, seq 31, length 64
E..T;...?..x..........p.5......c....e/...................... !"#$%&'()*+,-./01234567
22:37:02.917124 IP 192.168.29.152 > 192.168.26.152: ICMP echo request, id 13818, seq 32, length 64
E..T.V@.@...............5.. ...c............................ !"#$%&'()*+,-./01234567
22:37:02.917591 IP 192.168.26.152 > 192.168.29.152: ICMP echo reply, id 13818, seq 32, length 64
E..T?...?..-............5.. ...c............................ !"#$%&'()*+,-./01234567
^C
4 packets captured
4 packets received by filter
0 packets dropped by kernel
┌──[root@vms.154.liruilongs.github.io]-[~]
└─$
```
 ping 单通除了防火墙之外、网关是一个需要注意的地方。 
 
 ### Nginx 请求抓包分析用户名密码

安装 nginx ,配置nginx的用户认证

```shell
┌──[root@vms152.liruilongs.github.io]-[~]
└─$rpm -ql nginx || yum -y install  nginx
```
 备份修改配置文件 
 ```shell
┌──[root@vms.154.liruilongs.github.io]-[~]
└─$cp /etc/nginx/nginx.conf  /etc/nginx/nginx.conf.bak
┌──[root@vms.154.liruilongs.github.io]-[~]
└─$vim /etc/nginx/nginx.conf
```
 认证需要 在配置文件 server 模块下面添加对应的配置 
 ```shell
server {
        ...........
        auth_basic "auth-liruilong";
        auth_basic_user_file /etc/nginx/pass;
```

安装压测工具，http-tools可以创建访问网站的用户名和密码 
 ```shell
┌──[root@vms.154.liruilongs.github.io]-[~]
└─$yum -y install  httpd-tools
......
┌──[root@vms.154.liruilongs.github.io]-[~]
└─$htpasswd -c /etc/nginx/pass liruilong
New password:
Re-type new password:
Adding password for user liruilong
```
 启动服务 
 ```shell
┌──[root@vms.154.liruilongs.github.io]-[~]
└─$systemctl start  nginx
┌──[root@vms.154.liruilongs.github.io]-[~]
└─$systemctl is-active nginx
active
```
 访问抓包 
 ![](/home/attachment/202308/03/1263871_169102722258357.png)
 ```shell
┌──[root@vms.154.liruilongs.github.io]-[/var/log/nginx]
└─$tcpdump -i ens33 -A -w /root/web.cap host 192.168.26.1 and port 80
tcpdump: listening on ens33, link-type EN10MB (Ethernet), capture size 262144 bytes

^C67 packets captured
68 packets received by filter
0 packets dropped by kernel
```

通过抓包解析登录用户名密码 
 ```shell
┌──[root@vms.154.liruilongs.github.io]-[/var/log/nginx]
└─$tcpdump -A -r /root/web.cap | grep Basic
reading from file /root/web.cap, link-type EN10MB (Ethernet)
Authorization: Basic bGlydWlsb25nOmxpcnVpbG9uZw==
Authorization: Basic bGlydWlsb25nOmxpcnVpbG9uZw==
Authorization: Basic bGlydWlsb25nOmxpcnVpbG9uZw==
Authorization: Basic bGlydWlsb25nOmxpcnVpbG9uZw==
┌──[root@vms.154.liruilongs.github.io]-[/var/log/nginx]
└─$echo "bGlydWlsb25nOmxpcnVpbG9uZw==" | base64 -d \\\\n
liruilong:liruilong
┌──[root@vms.154.liruilongs.github.io]-[/var/log/nginx]
└─$
```

tcpdump 抓包 Linux

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